Japanese utility model publication (kokai) No. 1-166158 discloses a planetary differential gear system which comprises a differential casing provided with an external ring gear for receiving a drive input, and an internal ring gear disposed coaxially with the external gear, and rotatably supported by a fixed housing in a coaxial relationship, a sun gear rotatably supported inside the differential casing by the differential casing, and disposed coaxially with the internal gear, the sun gear being provided with a first splined bore coaxial with the sun gear for receiving a first axle shaft, engagement means for restricting a relative axial displacement between the first axle shaft and the sun gear, a carrier rotatably supported by the differential casing, carrying first planetary gears and second planetary gears both in a freely rotatable manner, the first planetary gears meshing with the internal gear, the second planetary gears meshing with the first planetary gears and the sun gear, the carrier being provided with a second splined bore coaxial with the sun gear for receiving a second axle shaft, and a clutch interposed between the carrier and the differential casing, and adapted to be engaged by a spring force of a dish spring which constantly applies an axial force to the clutch. Thus, the various gears provide a well known differential action, and the clutch constantly produces a force limiting the differential action. In other words, the differential action is limited by a constant degree at all times.
Japanese patent laid open publication (kokai) No. 4-95641 discloses a planetary center differential gear system for dividing a drive torque from an engine to front wheels and rear wheels. In this differential system, a pair of clutches are interposed on either axial end of each of the first planetary gears which are rotatably supported by the carrier. These clutches are interposed between the first planetary gears and the carrier. The gears consist of helical gears, and an axial reaction force is produced between each meshing pair of the helical gears. Therefore, when a torque is transmitted between the internal gear and the first planetary gears, the first planetary gears are forced in either axial direction depending on the direction of torque transmission. Thus, the differential action is limited whenever a drive torque or a braking torque is applied to the differential system. By appropriately determining the engagement forces of the two clutches on each first planetary gear, it is possible to produce two different properties of limiting the differential action depending on whether the engine is driving the wheels or the wheels are driving the engine (engine brake).
Japanese patent laid open publications (kokai) No. 4-231761 discloses a planetary differential gear system which is used to distribute power between right and left wheels. This planetary differential gear system is similar to the one disclosed in Japanese patent laid open publication (kokai) No. 4-95641 in that helical gears are used for producing reaction forces for actuating clutches adapted to limit the differential action, and two different properties of limiting the differential action can be produced depending on whether the engine is driving the wheels or the wheels are driving the engine. In this case, the two clutches are provided between the carrier and the differential casing, and between the sun gear and the differential casing, respectively. Also, because two sets of planetary gears are provided on the carrier, the axial reaction forces acting on them cancel out, and the reaction force is produced only in the sun gear. Thus, depending on the direction of torque transmission, either the carrier is urged in the direction to engage one of the clutches by the sun gear or the sun gear directly actuates the other clutch.
This planetary differential gear system allows the differential action to be limited depending on whether the engine is driving the wheels or the wheels are driving the engine. However, in reality, the spline coupling between the axle shaft and the splined bore of the sun gear does not allow free axial sliding movement between them particularly when a torque is being transmitted between them. In this differential gear system, it is essential that the helical gears, in particular the sun gear, can freely move in the axial direction for the frictional clutch to be able to limit the differential action of the differential gear system in a reliable fashion. Thus, if the axial movement of the helical gears is not effected in a smooth manner, the thrust acting on the frictional clutch may become unstable, and the differential action cannot be limited in a stable manner in particular in terms of a transient response.
Also, as the clutch facing wears out, the stroke of the sun gear necessary to engage the frictional clutch progressively increases. When the wear of the friction plates is excessive, metallic contacts may occur, and seizure of the mutually sliding members may even occur. In particular, the metallic contacts would produce highly undesirable noises which should be avoided. According to the invention disclosed in Japanese utility model publication (kokai) No. 1-166158), a limit is imposed on the axial movement of a pressure member in the direction to engage the clutch so as to avoid such metallic contacts.
In any case, it is necessary to provide means for controlling the possible range of movement of the sun gear to prevent any undesirable consequences from developing when the friction plates of the clutch have worn out.